Conventionally, after using a transmission belt for a long time, the belt body of the transmission belt is worn and the transmission belt is cracked at the surface due to hardening of the rubber of the belt. When the wear and the cracking of the transmission belt are advanced, the transmission belt can break at a tooth or another place, and the transmission power of the belt on a pulley can decrease remarkably or be lost. Therefore, the transmission belt needs to be exchanged before it is worn out or cracked.
However, wear or cracking of a transmission belt can not be easily recognized. Therefore, it is difficult to determine when the transmission belt will cease to transmit power, or by what amount the transmission power has decreased due to wear.
For example when a toothed belt is used for cam belt in a vehicle, the toothed belt is provided inside the engine therefore the wear and cracking of the toothed belt can not be checked. Namely, it is difficult to know when the transmission belt should be changed.
Therefore, as shown in Japanese Unexamined Utility Model Publication (KOKAI) NO. 60-12747, an apparatus composed of an electric circuit is shown, which warns when a transmission belt has reached the limit of its useful life. But the composition of this apparatus is very complex and it is difficult to put it to practical use.